Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 discloses a scroll type apparatus including two scroll mechanisms each having an end plate and spiroidal or involute spiral element. The scroll members are angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between the spiral surfaces, thereby sealing off and defining at least one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contact along the spiral surfaces to change the volume of the fluid pockets relative to the fluid outlet. The volume of the fluid pockets increases or decreases depending on the direction of the orbiting motion. A scroll type fluid displacement apparatus of this nature can be used to compress, expand or pump fluids.
In this type of fluid displacement apparatus, effective sealing of the fluid pockets is required. That is, axial and radial sealing of the fluid pockets must be maintained in order to achieve effective operation--the radial sealing being the line contact between the two interfitting spiral elements and the axial seal being the contact between the axial end surface of the spiral element and the inner end surface of the opposed end plate.
Various techniques have been used in the prior art to resolve the sealing problem, particularly, the axial sealing problem. For example, U.S. Pat. No. 3,994,636 discloses a technique for mounting a seal element in a groove in the free end of the spiral so that it can move freely. The seal element may be urged toward the opposed end plate by the resiliency of spring elements placed in the groove or by fluid pressure introduced into the groove from the fluid pockets. In this type of axial sealing, normally, the axial gap between the outer end surface of the spiral element and the inner surface of the opposed end plate is determined with respect to sealing the fluid pocket and the durability of seal element and the scroll. However, with a seal element loosely fitted within the groove, maintaing the axial gap is difficult because of the thermal expansion of the spiral element.
A further technique to resolve the axial sealing problem is disclosed in our earlier application Ser. No. 376,959 filed on May 11, 1982 now abandoned. In this prior device, the axial thickness of the seal element is greater than the depth of the groove and the seal element therefore extends between the bottom of the groove and the opposed end plate. However, in this type of sealing mechanism, the dimensions of the seal elements and the scrolls required a high degree of precision in establishing the axial gap. Thus, manufacturing of the scroll is complicated and relatively expensive.
Furthermore, even if the axial gap is correctly determined during the assembly of the compressor, the actual axial gap varies in operation because of the temperature change in the fluid as it is comprised, i.e., the temperature of the fluid at the center or fluid outlet portion of the scroll is higher than the temperature at the outer or fluid inlet portion of the scroll. The rate of thermal expansion of the spiral elements varies and, with a uniform axial gap, increased frictional contact between the end plate and spiral element would occur in the center of spiral element.